linux-kernel - Re: [Patch 0/2] Renaming 'trace' to 'relay' and enhancements to 'relay'

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Date:	Fri, 8 Aug 2008 09:22:39 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, dwilder@...ibm.com,
	hch@...radead.org, Steven Rostedt <rostedt@...dmis.org>,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [Patch 0/2] Renaming 'trace' to 'relay' and enhancements to
	'relay'

On Wed, Aug 06, 2008 at 11:08:10AM -0400, Frank Ch. Eigler wrote:
> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
> > [...]
> >> 	Please find the patches that enhance the 'trace' infrastructure
> >> (available in the -mm tree) and which introduce two new APIs
> >> relay_dump() and relay_printk().
> >> [...]
> 
> > I'm a bit perplexed by these trace patches
> > (http://userweb.kernel.org/~akpm/mmotm/broken-out/trace-code [...]
> > Is it useful?  Will it be useful?  [...]  I haven't heard much noise
> > about it and I'm struggling to justify merging it.
> 
> Right.
> 
> > Also, it's starting to look somewhat similar to ftrace, which also
> > provides sort of high-bandwidth per-cpu channels into userspace for
> > tracing purposes.
> 
> Perhaps ftrace ought to use this facility for its debugfs-facing bulk
> data interface rather than an internal one that cannot be used by
> anyone else.  Perhaps lttng could use it.  Systemtap could.  I believe
> a grand unification at this level was the idea.
>

Hi Andrew,
	The 'relay_printk' and 'relay_dump' interfaces were meant to
provide clutter-free tracing interface for the user who does not want to
care much beyond getting his trace output to user-space. It greatly
helps reduce the amount of work that a tracer needs to perform to setup
and tear-down. For e.g. when the Block I/O tracing code in
block/blktrace.c was converted to use these interfaces they resulted in
code-reduction of ~130 lines (http://lkml.org/lkml/2008/5/16/318).

The default values can work fine for most tunables and are also exposed
to the user for overriding them.

These interfaces will be helpful in almost all non-blocking tracers
(unlike usbmon which displays information in real-time) and uses the
scalable infrastructure provided by 'relay'.

As pointed out by Frank earlier, most tracers (including ftrace) can be
made to use the above-mentioned interfaces resulting in substantial
savings in terms of LoC and increasing modularity/code re-usability.

Thanks,
K.Prasad

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